In the prior art, designs of power supplies with sinusoidal outputs have used one of three techniques, including: (1) oscillator/amplifier arrangements, (2) synthesis from stored data, such as a ROM, or, more recently, a minicomputer, or (3) a large resonant tank circuit.
An example of an oscillator/amplifier system for producing a sinusoidal output is shown in U.S. Pat. No. 4,067,057 which issued Jan. 3, 1978 and is entitled "DC TO AC SWITCHING CONVERTER" by F. W. Taddeo and R. W. Pauly assigned to Pacific Electronic Enterprises, Inc. This circuit, while being a relatively simple circuit, relies heavily on the elimination of transformers and thus does not function as a true AC power supply since the AC voltage peak is limited to less than the DC input. The present invention is capable of providing an AC signal which is either above or below amplitude of the input voltage. Secondly, the prior art oscillator and amplifier arrangement requires a high quality local sinusoidal oscillator as part of the circuit while output distortion is dependent on the distortion of this oscillator. The present invention does not require a sinusoidal reference as the positive feedback loop functions to shape the sinusoidal output.
An example of a sinusoidal output which synthesizes the output from stored data may be found in U.S. Pat. No. 3,487,288 which issued on Dec. 30, 1969 and is entitled "PULSE WIDTH INVERTER" by H. Reid, Jr. and J. R. Currie assigned to the United States of America. This synthesizer suffers from the drawbacks of complexity and cost which are avoided by the present invention. Modulation consists of a resistive divider chain and a series of amplifiers, six in all, functioning as comparators whose outputs are selected as required according to the count of a self-setting clock. The scheme requires a complex clock, a programmed pulse selection system, and a pulse steering network all of which the present invention avoids. Additionally, he multiple amplifiers generate but one pulse width per amplifier while the present invention can generate a multiplicity of pulse widths from a single comparator found within the pulse width modulator. Finally, the device described in this prior art patent does not provide output voltage regulation to cope with line voltage variations or variations in load current as in the present invention. The present invention accomplishes regulation through a regenerative means of sine synthesis by referencing its own output in a bootstrap approach as will be described below.
A synthesizer for creating a sinusoidal output which incorporates a minicomputer may be found in U.S. Pat. No. 4,099,109 which issued on July 4, 1978 and is entitled "DIGITAL APPARATUS FOR SYNTHESIZING PULSE WIDTH MODULATED WAVEFORMS AND DIGITAL PULSE WIDTH MODULATED CONTROL SYSTEM" by A. Abbondanti which is assigned to Westinghouse Electric Corporation. The approach described in this reference uses a digital computer to synthesize the output waveform. The present invention provides a continuous output voltage over a substantially wide range of input voltages and output loads without the use of either a digital computer described in the '109 patent or stored data as described in the '288 patent.
Finally, a prior art system which utilizes a large resonant tank circuit may be found in U.S. Pat. No. 3,742,330 which issued on June 26, 1973 and is entitled "CURRENT-MODE DC TO AC CONVERTERS" by L. O. Hodges and L. R. Suelzle which is assigned to Data Electronic Control Corporation. This arrangement does not include a feedback from the load and thus operates in a quasi-open-loop mode. The present invention uses a closed-loop feedback that eliminates the need for an expensive and cumbersome output clamp as described in the '330 patent which is required because of the open-loop mode. Further, the present invention returns all unneeded power to the input rather than dissipating it as described within the '330 patent.
The subject matter of the present invention has been discussed in a publication which was authored by the inventors of this invention entitled "A New Technique For Sine Synthesis Inverter Design" which appeared in the Proceedings Of The Sixth National Solid-State Power Conversion Conference, May, 1979, POWERCON 6, published April, 1979, by Power Concepts, Inc. at pages F4-1 through F4-7.